1. Field of the Invention
The present invention relates to a method for transferring a dummy wafer made of SiC from a wafer cassette for holding it to a wafer boat when semiconductor wafers are heated in a wafer heating apparatus.
2. Discussion of Background
In manufacturing semiconductors, there are steps of forming a thin SiO2 film by the oxidation of a surface of a semiconductor wafer such as a silicon wafer polished to have a mirror surface, diffusing impurities such as phosphorus, nitrogen or the like and forming a thin Si3N4 or polysilicon film. For these steps, various kinds wafer treating apparatuses such as an oxidation apparatus, a diffusion apparatus, a LPCVD (low pressure chemical vapor deposition) apparatus or the like (hereinbelow, referred simply to heat treatment apparatuses) have been used.
As a heat treatment apparatus, there is, for example, a vertical type heat treatment apparatus 5 as diagrammatically shown in FIG. 1. A number of wafers to be treated are loaded on a wafer boat 20 capable of accommodating, for example, about 50-200 wafers. The wafer boat 20 is generally introduced into the apparatus from its lower portion as shown in the FIG. 1, and a heat treatment is conducted. The wafer boat is generally of a vertical type which comprises upper and lower disk-like fixing plates and three or four supporting columns each having a number of wafer supporting slots in which wafers are inserted to be supported, the supporting slots being formed with intervals of about several mm.
In such heat treatment apparatus, a flow of gas or temperature is apt to become uniform at both ends of the wafer boat. Accordingly, several wafers having the same shape as the semiconductor wafers are arranged as dummy wafers so that a uniform heat treatment is conducted to a number of wafers housed in the wafer boat.
Silicon wafers have generally been used as dummy wafers. In recent years, dummy wafers made of silicon carbide (hereinbelow, referred to as SiC dummy wafers) which lessens the consuming and the occurrence of particles due to the peeling of a film deposited on its surface, even in a highly elevated oxygen atmosphere have been gradually used. Since such dummy wafers are durable in a long term use, they are expected to contribute improvement of productivity. On the other hand, in industries, the transfer of wafers from a wafer cassette to a wafer boat is conducted with a transfer device (a transfer robot). The transferring operations include a step of taking correctly about 1-5 wafers by means of a wafer holding means of a loading device; moving the wafers to the wafer boat while they are held by the wafer holding means; inserting the wafers into the wafer boat through a narrow space between the supporting columns, and placing correctly the wafers in wafer supporting slots formed in the supporting columns. These steps should be carried out automatically and continuously without causing fracture or contamination of the wafers. In view of importance of and difficulty in these operations, various proposals have been made on the structure of the transfer device.
The operations of the transfer robot follow according to a programmed sequence previously inputted in a control device, for instance. In this case, however, it is preferable that the position of the wafer cassette and/or the position or the operation of the wafer boat are detected at predetermined positions by means of sensors such as photo-sensors located predetermined positions to detect an error to set values, and the operations are corrected by a feed-back control so that correct operations are performed according to the programmed sequence by means of, for example, servo mechanisms. In particular, administration of the transferred wafers is important. Therefore, the transferring operations are preferably conducted by detecting and/or monitoring, using sensors such as photo-sensors, the number of wafers which are taken from the cassette and held by the holding means of the transfer robot, the position of each of the wafers held by the holding means, the number of wafers placed regularly in the supporting slots in the wafer boat, and a state of arrangement and so on.
Visible light or an infrared ray is generally used as the light source for the photo-sensors. Since the conventional SiC wafers have high transmission properties to the visible light or the infrared ray, there is a big problem that the SiC dummy wafers do not respond to the sensors. Accordingly, when a dummy wafer is placed slightly shifted from a predetermined position in any step, such a slight shift can not be recognized as error whereby there is substantial reduction in reliability of the transferring mechanism installing sensors therein. Further, there is a possibility that a sensor erroneously responds to a dummy wafer (for example, when a dummy wafer is placed correctly, the sensor recognizes as being not placed), and the control device may operate to stop the operation of the transfer device. In this case, an operator had to do an appropriate treatment.
Further, from the reason of quality control, dummy wafers are loaded in a cassette inclusively used for the dummy wafers and wafers other than the dummy wafers are loaded in a cassette inclusively used for such wafers before they are supplied to the transferring steps. Then, the dummy wafers and the other wafers are taken from respective cassettes to be transferred to the same wafer boat, and are placed at predetermined positions in the wafer boat. After the heat treatment, the dummy wafers and the other wafers are separately taken from the wafer boat, and they are transferred to separate cassettes which are determined for inclusive use. Thus, although the dummy wafers and the other wafers are loaded on the same wafer boat, the transferring operations are conducted for the dummy wafers and the other wafers separately.
It is an object of the present invention to provide a method for transferring a dummy wafer of SiC wherein when a heat treatment is conducted to dummy wafers of SiC loaded in a wafer boat which is housed in a wafer heating apparatus, the transfer of the dummy wafer from a wafer cassette to the wafer boat is conducted by detecting or monitoring the number of dummy wafers and a state of arrangement of dummy wafers.
In accordance with the present invention, there is provided a method for transferring a dummy wafer of SiC from a wafer cassette for holding it to a wafer boat to be housed in a wafer heating apparatus by means of a wafer transferring device, the method being characterized in that the transferring device has means for conducting a step of taking a dummy wafer from the wafer cassette, a step of transferring the taken wafer to the wafer boat and a step of placing the transferred wafer in a wafer supporting slot in the wafer boat; the dummy wafer is of low transmission properties to light from the light source of a photo-sensor, and the transferring operation is carried out by detecting or monitoring wafers and a state of arrangement of the wafers using the photo-sensor at at least one of these steps.
Further, in accordance with the present invention, there is provided a method for transferring a dummy wafer of SiC from a wafer boat after a heat treatment to a wafer cassette for holding it by means of a wafer transferring device, the method being characterized in that the transferring device has means for conducting a step of taking a dummy wafer from the wafer boat, a step of transferring the taken wafer to the wafer cassette and a step of placing the transferred wafer in a wafer cassette; the dummy wafer is of low transmission properties to light from the light source of a photo-sensor, and the transferring operation is carried out by detecting or monitoring wafers and a state of arrangement of the wafers using the photo-sensor at at least one of these steps.
Each of dummy wafers of SiC (SiC dummy wafers) used in the present invention is a SiC dummy wafer having low light transmission properties. In the specification, the low light transmission properties mean that the transmittance of light having a wavelength used for the light source for a photo-sensor is low enough for detecting. In the case that the light source having a certain range of wavelength, the transmittance is measured at the wavelength wherein the photo-sensor is most sensible within the range. The transmittance of SiC dummy wafer is measured in perpendicular direction of the wafer. The transmittance is preferably 0.8% or less.
The wavelength of the light source for the photo-sensor is preferably 400-10000 nm, more preferably 400-5000 nm, more preferably 400-1500 nm.
With respect to how to produce the SiC dummy wafer having low transmission properties, there is in particular no imitation. However, it is preferable to use such a method that a SiC film of high purity is formed by coating according to a CVD method on a surface of a substrate such as a carbon substrate or a SiC substrate to thereby form, for example, a film comprising a plurality of SiC films. Namely, in the operations for forming the SiC film by CVD, film-forming conditions such as the flow rate of gas as raw material such as silane type, hydrocarbon type, silicon carbide type, halogenated silicon carbide type or the like, the flow rate of dilute gas such as hydrogen, helium, nitrogen or the like, the concentration of the raw material gas, reaction temperature, reaction pressure and so on are changed, for example, stepwise to thereby change the properties of crystal and/or the crystalline orientation, whereby the SiC film composed of at least two kinds of different crystal structures is formed.
When the SiC film formed by CVD has such different crystal structures or orientations, incident light is reflected, scattered and deflected in the dummy wafer whereby the light transmittance can be reduced. The different crystal structures or orientations may be in a form of layer or island. However, it is preferable for the crystalline structures or orientations to have a layered form from the viewpoint of easy production since the CVD coating method is basically a technique for forming a thin film. The combination of different crystal structures or orientations may be a combination of same xcex1-SiC having different orientations, a combination of same xcex2-SiC having different orientations or a combination of xcex1-SiC and xcex2-SiC.
There are many variations as to operating conditions of CVD to form such film of low transmission properties. For example, the flow rate of the raw material gas is periodically increased or decreased; the raw material gas is supplied intermittently or in a form of pulsation; reaction temperature and pressure are changed periodically or stepwise, or the kind or the flow rate of the dilute gas is changed periodically or stepwise. These operating conditions may be changed by combining at least two of the above-mentioned.
It is preferable that any dummy wafer on which the SiC film is formed by CVD, used in the present invention has substantially the same dimensions as any regular wafer such as a silicon wafer supported together with the dummy wafer in the same wafer boat. Namely, the dimensions of the dummy wafer are chosen in conformity with those of the regular wafer used. Usually, the thickness is 0.3-3.0 mm and the diameter (maximum) is 100-400 mm. Further, the dummy wafer may have a circular form or a form wherein a mark indicating an orientation is formed by cutting a part of the circular shape, an orientation flat form or a form having a notch.
In the present invention, a SiC dummy wafer or wafers of low transmission properties and a plurality of wafers which are respectively housed in separate wafer cassettes are respectively taken, and the SiC dummy wafer(s) and the regular wafers are transferred into a single wafer boat by means of a wafer transferring device.
The wafer transferring device is provided with means for conducting a step of taking a dummy wafer(s) from the wafer cassette, a step of transferring the taken wafer to the wafer boat and a step of placing the transferred wafer in a wafer supporting slot or slots in the wafer boat.